How does El Nino/La Nina impact Winter?

Issued Nov 26th 2012 1215 pm CST

In the weather business, one of the few good tools we have for seasonal forecasting is the El Nino Southern Oscillation, or ENSO. We look at sea water temperatures along the equator in the tropical Pacific Ocean. Although there are complex calculations for how we determine El Nino, the simple way to look at it is that if it is at least 0.5C (about 1F) warmer than normal for at least 6 months, then it is known as El Nino. If the waters are at least 0.5C (about 1F) cooler than normal for at least 6 months, it is known as La Nina. But what we rarely talk about is if it is close to normal, which is known as "Neutral".

Looking back at winters since 1950, these 3 conditions have occurred in nearly equal numbers. There have been 22 El Nino winters, 21 La Nina winters, and 20 Neutral winters during that time. Currently we are in ENSO Neutral conditions, and are no longer expecting an El Nino winter. The neutral conditions are forecast to continue through the winter and into the Spring.

We took a look at snowfall and temperatures associated with the various cycles of ENSO at Milwaukee since 1950.

During those 63 winters, the average snowfall at Milwaukee was 49.9" for 3 month meteorological winter of December, January, and February.

ENSO Cycle

Snowfall

%winters with above normal snowfall

All winters

49.9"

38% (4 in 10 winters above normal)

El Nino

44.0"

23% (1 in 4 winters above normal)

Neutral

51.7"

40% (4 in 10 winters above normal)

La Nina

53.4"

52% (1 in 2 winters above normal)

So there is a better chance of below normal snowfall in El Nino winters in Milwaukee, while La Nina winters tend to have the best chance of above normal snowfall. But, as we saw last winter, La Nina winters don't always have more than normal snowfall.

Now, looking at temperature, the average temperature for all 63 winters from 1950 to present(averaging both high and low temperatures) is about 24F in Milwaukee.

ENSO Cycle

Average Temperature

%winters with above normal temperature

All winters

23.9 F

46% (1 in 2 winters warmer than normal)

El Nino

24.5 F

50% (1 in 2 winters warmer than normal)

Neutral

23.0 F

40% (4 in 10 winters warmer than normal)

La Nina

24.2 F

48% (1 in 2 winters warmer than normal)

There is very little signal for temperature in Milwaukee winters using the ENSO cycle. Although you have some evidence that the warmer winters occur with El Nino and the colder winters occur with Neutral conditions, there is basically a coin toss probability of warmer versus colder than normal.

Since we remain in drought conditions, we also took a look at the total rainfall for the year based on what ENSO cycle we had during the winter. There was basically no signal, with each ENSO cycle having roughly a 50% chance of above or below normal rainfall for the whole year.

To summarize:

1) We expect ENSO Neutral conditions this winter, with tropical Pacific sea water temperatures close to normal

2) Since 1950, there have been 20 Neutral winters, which average 51.7" of snow at Milwaukee, about 2" above normal